1. Field of the Invention
The present invention relates generally to operational amplifiers and particularly to differential transconductance amplifiers that can monolithically integrated by CMOS technology.
2. Description of the Background Art
In recent years, LSIs and the like used to process an RF band signal in mobile communications employ a so-called Gm-C filter serving as a bandpass filter. This Gm-C filter is provided as a circuit having as its base an operational transconductance amplifier (hereinafter also referred to as an OTA circuit) performing voltage-current conversion. While this OTA circuit belongs in a broad sense to an operational amplifier, it also has a high output impedance for a closed loop, and is used for a variety of circuits for general purpose. Generally, this OTA circuit is arranged together with a so-called common mode feedback circuit and in response to the OTA circuit's differential output signal the common mode feedback circuit generates a common mode feedback control signal (hereinafter also simply referred to as a feedback control signal) and feeds the signal back to the OTA circuit. In response to feedback control signal fed back the OTA circuit has an output characteristic determined.
Japanese Patent Laying-Open No. 11-298271 discloses an amplifier capable of adjusting a feedback control signal.
The feedback control signal generated by the common mode feedback circuit is set to a value depending on a prescribed bias voltage, or a common mode feedback reference voltage (hereinafter also simply referred to as a feedback reference voltage).
Typically, this feedback reference voltage is set to have a prescribed designed value and provided as an operation reference voltage for a signal output from the OTA circuit.
However, as for example the OTA circuit's process varies, the OTA circuit has a constituent transistor varying in threshold voltage, β value and the like or in pinch-off voltage. Accordingly to allow the OTA circuit to achieve an optimal output characteristic it is necessary to adjust a feedback reference voltage as desired that is input. In the aforementioned reference, it is possible to simply adjust a voltage level. It is difficult, however, to consider the OTA circuit's process variation or the like to optimally adjust a voltage level.